fix: BGE-M3 and cross-encoder failing to load on HF Spaces

feat: Cross-encoder search reranking with MiniLM-L-6

Phase 6.5: Pipeline telemetry, search UX fixes, latency profiling

Phase 6.5 Day 5: Semantic Scholar author import (B4)

Phase 5: Cold-Start Onboarding + UI Redesign

Phase 3.5: Wire Turso DB for metadata (2.9x faster, includes citations)

Use python-dotenv for secrets management

Phase 3 complete: Hybrid Semantic Search pipeline